Viral infections are known to be one of the most frequent causes of human illness. Upwards of 300 different immunologic types of virus have been associated with humans; however, not all of these are identified with clinical recognizable diseases.
Many of these diseases, having once been acquired, render their host free from further infections by the same agent by stimulating a life-long immunologic response by the host to the viral agent. It is by this very same mechanism that conquest of certain virus-caused disease states has been achieved. By using prophylaxis induced by either killed or attenuated viruses or infection with an immunologically related virus which causes a very mild disease state, vaccines have been developed, for smallpox, yellow fever, polio, and some of the common childhood diseases, e.g. mumps, rubella, and measles.
While prophylaxis from some viral diseases can be obtained by immunity, immunologic protection from other viral diseases is not possible because either prolonged immunity is not developed against the virus, or the same clinically described disease is caused by a large group of related viruses which are antigenically dissimilar and do not produce cross immunity, e.g., common cold viruses.
Coupled with this lack of universal prophylaxis against all viral diseases is the necessity to effect a cure in an already established viral disease. In search for this cure of viral diseases, several chemotherpeutic agents have been used. Included in this group are amantadine (1-adamantanamine), methisazone (1-methylisatin B-thiosemicarbazone), cytarabine (cytarabine (cytosine arabinoside), 5-IDU (5-iodo-2'-deoxyuridine); however, these agents are of either limited spectrum, e.g., amantadine is only active against Type A influenze virus, cytarabine and 5-IDU are not active against RNA viruses and they are quite toxic.